1. Field of the Invention
The present invention relates to a plasma processing apparatus capable of suppressing a damage due to sputtering to a wall surface of a processing chamber during plasma occurrence.
2. Description of the Related Art
Conventionally, there are two types of plasma processing apparatuses. One type generates plasma by applying high-frequency power with a single type of frequency. The other type generates plasma by applying high-frequency power with two types of frequency. Especially, the latter type of plasma processing apparatus is configured to generate high-density plasma using high-frequency power with a high frequency, and generate a bias potential using high-frequency power with a low frequency. In recent years, there is often used a plasma processing apparatus which alternates the two types of frequencies according to necessity.
As shown in FIG. 10, this plasma processing apparatus comprises upper and lower electrodes 2a and 2b, and first and second high-frequency power supplies 3a and 3b. The upper and lower electrodes 2a and 2b are arranged parallel facing each other in a processing chamber 1. The first and second high-frequency power supplies 3a and 3b supply these upper and lower electrodes 2a and 2b with first and second high-frequency powers having different frequencies.
A matching circuit 5a is provided in the middle of a first power supply line 4a connecting the first high-frequency power 3a and the upper electrode 2a. A matching circuit 5b is provided on a second power supply line 4b connecting the second high-frequency power 3b and the lower electrode 2b. 
Further, the first power supply line 4a connects with a first filter circuit 6a as a return circuit. This filter circuit 6a has a fixed circuit constant and attenuates a second high-frequency component. The second power supply line 4b connects with a second filter circuit 6b as a return circuit. This filter circuit 6b has a fixed circuit constant and attenuates a first high-frequency component. In the first and second filter circuits 6a and 6b, each circuit constant is set to a value resonating with a high frequency to be filtered.
When plasma processing is applied to a processing object such as a semiconductor wafer, the processing object is mounted on the lower electrode 2b, for example. The first and second high-frequency power supplies 3a and 3b supply corresponding high-frequency powers to the upper and lower electrodes 2a and 2b, respectively, to generate plasma P therebetween. Further, a bias voltage is applied to the lower electrode 2b to perform specified plasma processing for the semiconductor wafer on the lower electrode 2b. 
The plasma processing apparatus attenuates high-frequency power with a different frequency on the first and second power supply lines 4a and 4b. The first and second high-frequency power supplies 3a and 3b supply respective high-frequency powers to the upper and lower electrodes 2a and 2b under an optimum condition.
In this technology, however, a potential difference occurs between the plasma potential and the wall surface of the processing chamber 1. Sputtering due to an ion component in the plasma erodes the wall surface. Simultaneously applying two types of high-frequency power decreases an effective ground area for one electrode compared to the case where a single frequency is applied. A sputter rate increases for the decreased ground area, causing more damages of the wall surface. This has been a cause of the shortening of the processing chamber life (usable period).
The problem of shortening the processing chamber life becomes more serious when the processing object such as a semiconductor wafer has an increased area, throughput is increased, or the high-frequency power is increased to further increase the sputter rate, and the like.
Conventionally, countermeasures have been taken to suppress damages to the wall surface. For example, the processing chamber is enlarged to increase the ground area and decrease the sputter rate. Alternatively, a resin coating is applied to the wall surface to prevent wear to the wall surface. In any case, substantial countermeasures is unavailable, causing a problem of increasing costs for additional processing.